1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the semiconductor device.
2. Description of the Related Art
An active matrix substrate for use in a liquid crystal display device, or the like, includes a switching element, such as a thin film transistor (hereinafter, “TFT”), in each pixel. Examples of such a switching element which have been conventionally used in various applications include a TFT which includes an amorphous silicon film as an active layer (hereinafter, “amorphous silicon TFT”) and a TFT which includes a polycrystalline silicon film as an active layer (hereinafter, “polycrystalline silicon TFT”).
Since the mobility of electrons and holes in the polycrystalline silicon film is higher than in the amorphous silicon film, the polycrystalline silicon TFT has a higher ON current, and is capable of operating at a higher speed, than the amorphous silicon TFT. Thus, when an active matrix substrate is fabricated using the polycrystalline silicon TFT, the polycrystalline silicon TFT can also be used for a peripheral circuit, such as a driver and the like, as well as the switching elements. Therefore, there is an advantage that part or entirety of the peripheral circuit, such as a driver and the like, and a display portion can be integrally formed on the same substrate. Also, there is another advantage that the pixel capacitor of a liquid crystal display device, or the like, can be charged within a shorter switching time.
However, fabrication of the polycrystalline silicon TFT requires complicated steps, including a laser crystallization step for crystallization of the amorphous silicon film as well as a heat anneal step, an ion doping step, etc. These steps disadvantageously increase the fabrication cost per unit area of the substrate. Therefore, the polycrystalline silicon TFT has been mainly used in middle-size and small-size liquid crystal display devices.
On the other hand, the amorphous silicon film can be formed more easily than the polycrystalline silicon film and is therefore suitable to larger display sizes. Thus, the amorphous silicon TFT is suitably used in an active matrix substrate of a device of which a large area is demanded. In many active matrix substrates for liquid crystal television displays, the amorphous silicon TFT is used, notwithstanding it has a lower ON current than the polycrystalline silicon TFT.
However, when the amorphous silicon TFT is used, improvement in performance is limited because the mobility is low in the amorphous silicon film. Particularly, in recent years, improvement in display quality and reduction in power consumption as well as increase in display size have been highly demanded of liquid crystal display devices, such as liquid crystal television displays. The amorphous silicon TFT has a difficulty in sufficiently meeting such a demand.
To realize a TFT of higher performance with a reduced number of fabrication steps and a reduced fabrication cost, uses of other materials than amorphous silicon and polycrystalline silicon for the material of the active layer have been attempted. Patent Document 1, Patent Document 2, and Non-patent Document 1 propose that a microcrystalline silicon (μc-Si) film is used to form an active layer of a TFT. Such a TFT is called “microcrystalline silicon TFT”.
The microcrystalline silicon film is a silicon film which contains microcrystalline grains in its inside. The grain boundary of the microcrystalline grains is mainly composed of an amorphous phase. Specifically, the grain boundary is in a state where the microcrystalline grains and the amorphous phase are mixed. The size of each microcrystalline grain is smaller than the size of the crystal grains contained in the polycrystalline silicon film. Also, as will be described in detail later, in the microcrystalline silicon film, each microcrystalline grain has the shape of, for example, a column grown from the substrate surface.
The microcrystalline silicon film can be formed only by a film formation step with the use of a plasma CVD method. As the material gas, a silane gas diluted with a hydrogen gas can be used. In the case of forming a polycrystalline silicon film, after formation of an amorphous silicon film with the use of a CVD apparatus or the like, the step of crystallizing the amorphous silicon film by laser or heat (annealing step) is necessary. On the other hand, in the case of forming a microcrystalline silicon film, a microcrystalline silicon film which includes a basic crystalline phase can be formed by a CVD apparatus or the like, and thus, the annealing step with laser or heat can be omitted. Since the microcrystalline silicon film thus can be formed by a smaller number of steps than that required for the formation of the polycrystalline silicon film, the microcrystalline silicon TFT can be fabricated with substantially the same degree of productivity as that of the amorphous silicon TFT, i.e., with substantially the same number of steps and substantially the same fabrication cost. Also, a microcrystalline silicon TFT can be fabricated using an apparatus designed for fabrication of the amorphous silicon TFT.
Since the microcrystalline silicon film has a higher mobility than the amorphous silicon film, using the microcrystalline silicon film can realize a higher ON current than the amorphous silicon TFT. Since the microcrystalline silicon film can be formed without complicated steps as required in the formation of the polycrystalline silicon film, larger areas can easily be realized.
Patent Document 1 describes using a microcrystalline silicon film as the active layer of a TFT, whereby the ON current achieved is 1.5 times that achieved in an amorphous silicon TFT. Non-patent Document 1 describes using a semiconductor film composed of microcrystalline silicon and amorphous silicon, whereby a TFT is obtained in which the ON/OFF current ratio is 106, the mobility is about 1 cm2/Vs, and the threshold is about 5 V. This value of the mobility is equal to or greater than that of the amorphous silicon TFT. Note that the TFT described in Non-patent Document 1 includes an amorphous silicon layer which is provided on a microcrystalline silicon layer in order to reduce the OFF current.
Patent Document 2 discloses an inverted staggered TFT in which microcrystalline silicon is used.    Patent Document 1: Japanese Laid-Open Patent Publication No. 6-196701    Patent Document 2: Japanese Laid-Open Patent Publication No. 5-304171    Non-patent Document 1: Zhongyang Xu et al., “A Novel Thin-film Transistors With μc-Si/a-Si Dual Active Layer Structure For AM-LCD” IDW'96 Proceedings of The Third International Display Workshops VOLUME 1, 1996, pp. 117-120.